IMPDH (EC 1.1.1.205) is an enzyme involved in the de novo synthesis of guanosine nucleotides. The synthesis of nucleotides in organisms, in general, is required for the cells in those organisms to divide and replicate. Nucleotide synthesis in mammals may be achieved through one of two pathways: the de novo synthesis pathway or the salvage pathway. Different cell types use these pathways to different extents. IMPDH catalyzes the NAD-dependent oxidation of inosine-5′-monophosphate (IMP) to xanthosine-5′-monophosphate (XMP) [Jackson R. C. et. al., Nature, 256, pp. 331-333, (1975)].
IMPDH is ubiquitous in eukaryotes, bacteria and protozoa [Y. Natsumeda & S. F. Carr, Ann. N.Y. Acad., 696, pp. 88-93 (1993)]. The prokaryotic forms share 30-40% sequence identity with the human enzyme. Two isoforms of human IMPDH, designated type I and type II, have been identified and sequenced [F. R. Collart and E. Huberman, J. Biol. Chem., 263, pp. 15769-15772, (1988); Y. Natsumeda et. al., J. Biol. Chem. 265, pp. 5292-5295, (1990)]. Each is 514 amino acids, and they share 84% sequence identity.
The de novo synthesis of guanosine nucleotides, and thus the activity of IMPDH, is particularly important in B and T-lymphocytes. These cells depend on the de novo, rather than salvage pathway to generate sufficient levels of nucleotides necessary to initiate a proliferative response to mitogen or antigen [A. C. Allison et. al., Lancet 2(7946), pp. 1179-1183, (1975) and A. C. Allison et. al., Ciba Found. Symp., 48, pp. 207-224, (1977)]. Thus, IMPDH is an attractive target for selectively inhibiting the immune system without also inhibiting the proliferation of other cells.
In addition to its role in the immune response, it is also known that IMPDH plays a role in other metabolic events. Increased IMPDH activity has been observed in rapidly proliferating human leukemic cell lines and other tumor cell lines, indicating IMPDH as a target for anti-cancer as well as immunosuppressive chemotherapy [M. Nagai et. al., Cancer Res., 51, pp. 3886-3890, (1991)]. IMPDH has also been shown to play a role in the proliferation of smooth muscle cells, indicating that inhibitors of IMPDH, such as MPA or rapamycin, may be useful in preventing restenosis or other hyperproliferative vascular diseases [C. R. Gregory et al., Transplantation, 59, pp. 655-61 (1995); PCT publication WO 94/12184; and PCT publication WO 94/01105].
Additionally, IMPDH has been shown to play a role in viral replication in some viral cell lines. [S. F. Carr, J. Biol. Chem., 268, pp. 27286-27290 (1993)]. Analogous to lymphocyte and tumor cell lines, the implication is that the de novo, rather than the salvage, pathway is critical in the process of viral replication.
Mycophenolic acid (MPA) and some of its derivatives have been described as inhibitors of IMPDH [U.S. Pat. Nos. 5,380,879 and 5,444,072 and PCT publications WO 94/01105 and WO 94/12184]. These compounds are potent, non-competitive, reversible inhibitors of human IMPDH type I (Ki=33 nM) and type II (Ki=9 nM). MPA has been demonstrated to block the response of B and T-cells to mitogen or antigen [A. C. Allison et. al., Ann. N.Y. Acad. Sci., 696, pp. 63-87, (1993)]. MPA is characterized by undesirable pharmacological properties, however, such as gastrointestinal toxicity and poor bioavailability. [L. M. Shaw, et. al., Therapeutic Drug Monitoring, 17, pp. 690-699, (1995)].
Mycophenolate mofetil (MMF), a prodrug which quickly liberates free MPA in vivo, has been approved to prevent acute renal allograft rejection following kidney transplantation. [L. M. Shaw, et. al., Therapeutic Drug Monitoring, 17, pp. 690-699, (1995); H. W. Sollinger, Transplantation, 60, pp. 225-232 (1995)]. Several clinical observations, however, limit the therapeutic potential of this drug. [L. M. Shaw, et. al., Therapeutic Drug Monitoring, 17, pp. 690-699, (1995)]. First, the active drug, MPA, is rapidly metabolized to the inactive glucuronide in vivo. [A. C., Allison and E. M. Eugui, Immunological Reviews, 136, pp. 5-28 (1993)]. The glucuronide then undergoes enterohepatic recycling causing accumulation of MPA in the gastrointestinal tract where it cannot exert its IMPDH inhibitory activity on the immune system. This effectively lowers the drug's in vivo potency, while increasing its undesirable gastrointestinal side effects. In addition, MMF has inherent drawbacks as a prodrug. MMF is the morpholinoethyl ester of MPA. In vivo MMF is deesterified to MPA, but this hydrolysis can occur over a wide pH range in an aqueous environment. Therefore, it is difficult to control the time and location of activation of the drug.
Urea derivatives, which are more effective than MPA as inhibitors of IMPDH, have recently been described in U.S. Pat. No. 5,807,876 and in co-pending continuation application Ser. Nos. 08/801,780 and 08/832,165, herein incorporated by reference. These compounds exhibit both an increased overall therapeutic effect and decreased deleterious side effects in their inhibition of IMPDH and in their use as compositions. But the aqueous solubility of these compounds is less than optimum.
The aqueous solubility of an organic molecule can impact its absorption following oral administration. For example, the oral administration of a highly hydrophobic compound can very easily result in poor absorption due to precipitation in the gastrointestinal tract. Formulation of such hydrophobic compounds with surfactants and complexing agents can improve the aqueous solubility of these compounds, but this method becomes more impractical as the aqueous solubility decreases. However, chemical modification of a drug into a bio- or chemically-reversible prodrug can confer temporary aqueous solubility to the drug substance that allows absorption following oral administration.
For orally administered prodrugs, the drug substance's kinetic solubility in neutral to acidic media is of most interest. In most cases, the kinetic solubility in these media is higher than the corresponding thermodynamic solubility. Therefore, it is advantageous to utilize this transient increase in solubility that immediately follows the conversion of the prodrug to the drug substance. The time it takes to reach thermodynamic equilibrium will vary from compound to compound and can only be determined experimentally. A strategy for creating prodrugs of IMPDH inhibitors that exploits the compound's kinetic solubility would be advantageous. Alternatively, a prodrug which liberates the drug substance as a fine dispersion intestinally may also improve its oral absorption, with smaller particle sizes being preferred.
Prodrug strategies which rely on intramolecular cyclization/transacylation to liberate a drug substance and a lactam derivative have been described where the liberated drugs are alcohols, phenols, and primary and secondary amines. For alcohols [Saari, et al, J. Med. Chem., 33, pp. 2590-2595 (1990)] and phenols [Saari, et al, J. Med. Chem., 33, pp. 97-101 (1990)], the facility of cyclization and hydroxyl liberation is a consequence of the lower pKa of the resulting leaving group (pKa 10-16). Such prodrugs are easily prepared by known methods which offer a reasonable amount of synthetic flexibility allowing one to modulate the rate of prodrug to drug conversion. The rates of liberation for cyclizing alcohol and phenol prodrugs are sensitive to pH. For amines [Borchhardt, et al, Pharm. Sci., 86, pp. 765-767 (1997)] strategies have been developed which utilize cyclization of highly constrained systems as well as the use of additional functionalization in the form of animals. Such measures are taken to overcome the poor ability of alkyl amines to serve as leaving groups (pKa≧30). These methods, however, are inadequate for the formation of prodrugs of drugs lacking alcohols, phenols, or primary and secondary amines.
Thus, there is a need for prodrugs of potent IMPDH inhibitors. Desirable properties of these prodrugs would include better aqueous solubility with corresponding improved bioavailability, and the ability to be activated at particular times and locations in the body as needed. Such prodrug inhibitors would have therapeutic potential as immunosuppressants, anti-cancer agents, anti-vascular hyperproliferative agents and anti-viral agents. Specifically, such compounds may be used in the treatment of transplant rejection and autoimmune diseases, such as rheumatoid arthritis, multiple sclerosis, juvenile diabetes, asthma, inflammatory bowel disease, as well as in the treatment of cancer and tumors, such as lymphomas and leukemia, vascular diseases, such as restenosis, and viral replication diseases, such as retroviral diseases and herpes.